(1) Technical Field
This invention relates to scanning electron microscopy and more particularly to producing image exactness during the measurement of critical dimensions pertaining to semiconductor structures.
(2) Description of the Prior Art
The following documents describes external shielding for an scanning electron microscope (hereinafter referred to as SEM)
U.S. Pat. No. 6,043,491 issued Mar. 28, 2000 to Yoichi Ose et al., uses external shielding for clearer SEM images by suppressing interferences.
U.S. Pat. No. 5,591,971 issued Jan. 7 1997 to Shahar et al., and U.S. Pat. No. 5,229,607 issued Jul. 20, 1993 to Matsui et al., show related patents describing an interposing mesh structure for preventing electrical field leakage during SEM imaging.
U.S. Pat. No. 4,907,287 issued Mar. 6, 1990 to Homma et al., describe a SEM that doesn""t need extra shielding
A scanning electron microscope (SEM) is widely used in the semiconductor industry for measurement at very high magnification, providing indisputable data for optimizing, for example, correctness of critical dimensions of structures being formed on semiconductor devices. In a SEM, a specimen is irradiated with a focused beam of electrons directed to scan a plane of the specimen. The analysis is done by moving the focused beam of electrons in a sweeping (raster) action over the plane of the specimen. Electrons are scattered from the sample as the beam traverses it, a process known as proximity effect, or back scattering, X-rays are also emitted as the electrons are precipitately stopped after hitting the sample. These interactions are well understood by those knowledgeable with the situation. The scattered and emitted electrons are collectively processed from suitable detectors, the output of which, are combined to provide a signal depicting the topography of the scanned sample. This signal is converted into an intensity modulated input into a CRT for display, thus obtaining a scanned image (SEM image) of the specimen for analysis.
Other methods for image correction which appear as superposed disturbances in a SEM image while under the influence of vibration is reduced by averaging the image data over a plurality of image frames, and further correcting image distortion produced under action of magnetic fields generated by power sources employed in peripheral equipment described in the referenced prior art. U.S. Pat. No. 4,907,287. However, competitive pressures in the semiconductor industry require manufacturers to achieve maximum throughput rates in their production lines.
The SEM is utilized for an observation or measurement of a submicron size feature formed in a wafer specimen for a semiconductor device. In such an application, it is required that the SEM allow measurement to be made with a  less than 10 nm of resolution. Moreover, the SEM critical measurement must be automated, accurate, and fast Generally between 5 and 10 target locations on each semiconductor wafer are measured in this way. Measurement must be done at rates exceeding 25 wafers per hour or between 125 to 250 targets per hour, permitting 14 to 28 seconds per target. This includes wafer handling and alignment times.
The present invention concerns an image enhancement device which can be advantageously used for reducing distortions appearing in a scanned image. The distortions are influenced by an externally generated electro-magnetic emission signal from a component of alternating current. The enhancement is accomplished without a need for averaging image data over a plurality of image frames.
It is therefore an object of the present invention to eliminate the drawbacks of the prior art system described above, and provide an image enhancement device for a scanning electron microscope (SEM) that produces an output image of exactness and quality without averaging image data over a plurality of image frames.
It is another object of the present invention to improve on the image quality thereby ameliorating the 3-sigma uncertainty in the SEM image (line) measurement results.
In view of the above objects, it is proposed according to the present invention that distortion appearing on the image which is influenced by an externally generated electro magnetic emission signal be eliminated without added filtering algorithms or the like. More specifically, in accordance with the present invention, predictability and stability of the image quality are accomplished by mounting an image compensation device to the column housing of a scanning electron microscope, for example, an SEM which is used exclusively for measuring critical dimensions of a specimen. The image compensation device introduces a known electro-magnetic, 50 Hz. A/C component within the shielding coil of the SEM, therein, nullifying the effects of the externally generated electro magnetic emission, hence, producing a distortion free image quality.